What remains one of the most fascinating areas in biological research is the mechanism regulating the events of aging. The main theme of this proposal suggests that a defined genetic program is in control of the irreversible termination of cell replication as cells become senescent. We have hypothesized that normal fibroblasts are destined for senescent nonproliferation as the initial part of a final program leading to death. Therefore senescence similar to terminal differentiation, may be a state, obligatory and dominant in every cell, and distinctly different from the transient growth-arrest seen in quiescence. Here we hypothesize that a specific program of gene expression sis activated for cells to become nonproliferative and senescent. The first step of our research along this line is to identify gene(s) or their product(s) uniquely present in senescent cells and absent in their young growing or nongrowing counterparts. We have succeeded in identifying a first example of such a gene product, terminin. Identification of terminin provides a handle to test our hypothesis by performing experiments to: (1) characterize terminin presence in tissues by immunohistochemical studies; (2) assess terminin expression qualitatively and quantitatively in tissues derived from young, middle-aged, and old animals; (3) characterize the cellular identity of terminin-positive granules by immunogold ultrastructural studies and histochemical enzymatic assays; (4) characterize possible biochemical mechanisms regulating terminin expression; (5) purify terminin polypeptides, sequence the protein and generate macromolecular probes; and (6) carry out initial cloning and sequencing of cDNA clones encoding for terminin proteins. Results of these experiments will provide us the necessary information and tools to investigate the next series of questions on how terminin gene expression is controlled and what is the function of the protein relating to the aging process. Ultimately, it will shed light on the mechanism of how cells turn off replication permanently as seen in fibroblasts senescence or terminal differentiation, the last part of a long physiological process preparing for the final event of apoptosis, the destiny of every cell.